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A case of extensor digitorum brevis manus

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A case of extensor digitorum brevis manus
Mineo Nakano1, Yumiko Watanabe2, Mamoru Masutani2
Dermatology Online Journal 9 (5): 21

From the Division of Plastic and Reconstructive Surgery1 and Department of Dermatology2, Organized Center of Clinical Medicine, International University of Health and Welfare, Tochigi, Japan.


The extensor digitorum brevis manus (EDBM), a relatively rare anomalous muscle on the dorsal hand, may be misdiagnosed as a ganglion, a synovial nodule or cyst, or a soft-tissue tumor. MRI scans can help to distinguish EDBM from tumors. EDBM should be included in the differential diagnosis of soft tissue masses on the dorsal aspect of the hand.

Approximately 295 cases of extensor digitorum brevis manus (EDBM) have been reported since this anomalous muscle was first described by Albinus in 1734. However, to our knowledge there has been no previous report of this entity in the dermatologic literature. We present a case of EDBM confirmed by magnetic resonance imaging (MRI) scans, and we discuss the difficulties in making a clinical diagnosis, the anatomical findings, and the treatment of EDBM.

Case report

A 30-year-old, right-handed man presented with a painful mass in the dorsal aspect of the right wrist. He had noticed a mass on the dorsal aspect of each wrist in his junior high school days. Initially the masses were painless, but pain occurred several months after he started to do heavy manual work

Figure 1
Prominent masses on the dorsum of both hands.

Physical examination revealed a fusiform, soft prominence on each dorsal wrist between the extensor tendons of the index and middle fingers when the fingers were fully extended against resistance. A dull pain was noted with movement or palpation of these masses. At first, they were thought to be ganglions, synovial tumors, or tenosynovitis of the extensor tendons. Because an aspiration test was negative, a biopsy was performed under local anesthesia to reach an accurate diagnosis. During surgery, an anomalous muscle was found, and an incisional biopsy was performed. The histologic specimen showed normal striated muscle. In addition, bilateral EDBM were confirmed by MRI scans (figs. 2a, 2b, and 2c).

Figure 2a Figure 2b
Axial T2* weighted MRI of the same hands confirming the presence of EDBM. Axial T2* weighted (47/12/50, TR/TE/Flip Angle) MRI image shows an ovoid homogeneous mass of intermediate signal intensity surrounding the extensor digitorum communis extensor tendons of the index finger and long fingers of the right hand (arrow), lying at the level of the carpometacarpal joints (fig. 2a).
Axial T2* weighted (20/12/50, TR/TE/Flip Angle) MRI shows a similar mass albeit rather flatter and smaller than right-hand mass, located between the extensor tendons of the index and long fingers of the left hand (arrow)(fig. 2b).

Figure 2c
Axial T2* weighted (20/12/50, TR/TE/Flip Angle) MRI shows that both EDBM bellies extended to the mid-portion of the third metacarpal bone, as confirmed by the clinical and MRI findings (the presentation is left hand).

T2* weighted axial images showed an ovoid mass of intermediate signal intensity, similar to normal muscle, surrounding the extensor digitorum communis tendons of the index finger and long finger of the right hand. A similar but flatter and smaller mass was located between the extensor tendons of the index and long fingers of the left hand.

No further surgery was carried out because the dull pain was relieved after we advised him to reduce the amount of heavy manual work he performed.


Anomalous extensor muscles of the hand are relatively common, and many have been reported in surgical publications. The EDBM, which is one of these muscles, was described first by Albinus in 1734 [1]. Since then, 295 cases (including human cadavers and living subjects) have been reported, and the prevalence is estimated to be 1-2 percent [1, 2]. It occurs bilaterally in approximately one third of cases, and there is no difference in incidence between the right and left hands or between the genders. The origin of EDBM has been described as being at the distal end of the radius, the dorsal radiocarpal ligament, or the wrist-joint capsule [3, 4]. Its insertion has been described as being in the extensor hood of the index, middle, ring, or little finger, although multiple insertions into more than one finger may occur. The most common insertion is into the index finger, followed by the middle, and then the index and middle fingers [1]. The innervation and blood supply of EDBM has been confirmed as the posterior interosseous nerve and artery. EDBM becomes more prominent and firm with resisted finger extension. EDBM usually causes little or no pain, although resistive extension of the finger may cause pain in the hands containing the EDBM, and pain may be produced by the patient pushing the palm against the table with the wrist extended. The pain is likely to be a consequence of muscle hypertrophy and of impingement of the muscle against the extensor retinaculum [3, 5]. Heavy use of the hand may lead to pain during the most active period in the patient's life, and when EDBM is present bilaterally the symptoms usually occur in the dominant hand. In our case, the symptoms resulting from the EDBM were consistent with those previously described, and the muscle belly was observed to extend to the midportion of the third metacarpal bone (from the clinical and MRI findings).

We first made the correct diagnosis at the time of surgery. Preoperatively EDBM is often confused with a ganglion, a synovial condition (tenosynovitis, synovial tumor, tendon sheath cyst), other benign soft tissue tumors, carpal bossing, or exostosis. Moreover, ganglions associated with EDBM have been reported [2, 3, 6], and this occurrence may make a correct diagnosis of EDBM more difficult.

An electrophysiologic study using surface electrodes can establish a preoperative diagnosis of EDBM [5], although this test may be rather cumbersome. In recent years, noninvasive MRI scans have been found to be helpful in distinguishing EDBM from tumors, thereby avoiding surgery. MRI scans usually display EDBM as an intermediate- to low-density signal with extreme homogeneity on both T1- and T2-weighted images, an appearance similar to that of normal skeletal muscle [1, 7].

Treatment depends on the severity of the symptoms. Conservative treatment with short-wave diathermy, paraffin baths, immobilization, or anti-inflammatory drugs has been found to be unsuccessful when the symptoms are severe. Although division of the retinaculum may provide symptomatic relief, some surgeons have recommended muscle excision because retinacular release often fails to relieve the pain. Indeed, a previous review showed that 75 percent of cases required muscle excision [8]. However, resection of EDBM should be avoided in cases where EDBM is compensating for the absence of the extensor indicis proprius [1, 2, 3, 5].

EDBM should be included in the differential diagnosis of soft tissue masses on the dorsal aspect of the wrist and hand and may be diagnosed by MRI.

Acknowledgment.—The authors thank Katsuyoshi Hirota for his technical assistance with the MRI.


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